Preheat your oven to 350F/ 180C and line a loaf pan with parchment paper. To make the walnut crumb, add ¼ cup almond flour, crushed walnuts, melted coconut oil, and powdered erythritol to a bowl. Mix to combine, and set the bowl aside.
To a bowl, add the bananas and mash them until soft. Add the oil, eggs, coconut yogurt, coconut milk, vanilla extract, and banana extract. Whisk until incorporated.
To another bowl, add 1 ¾ cup almond flour, granulated erythritol, baking soda, cream of tartar, cinnamon, and salt. Whisk to combine. Pour the wet ingredients into the dry ingredients and fold until incorporated.
Pour the batter into the lined loaf pan. Crumble the walnut crumb dough over the batter. Transfer the pan to the oven.
Bake for 45-60 minutes, or until an inserted toothpick comes out dry. If the crumb starts browning too fast, cover it with aluminum foil. Take the pan out of the oven and place it on a cooling rack.
Cut the banana bread into 12 slices. Serve immediately or store it for later. Serve it on its own or with a cup of coffee or tea.